Due to their advantageous low power consumption and lack of toxic materials, solid state light emitting diode (LED) lighting applications are rapidly replacing conventional incandescent and florescent lighting systems. However, an LED cannot be exposed to the AC mains like an incandescent bulb. Solid state lighting applications thus include a switching power converter to convert the AC input current into a rectified DC output current that may power the LED. A controller controls a power switch in the switching power converter so that the desired current powers the LED. The controller needs its own power supply voltage, which is designated herein as VCC. The generation of VCC for the controller must balance cost and efficiency. The need for efficient generation of VCC also applies to other types of switching power supplies such as an AC-DC adapter and charger.
The generation of VCC depends upon the switching power converter architecture. For example, the power switch in a flyback power couples to a primary winding of a transformer. An auxiliary winding on the transformer thus provides a convenient and very efficient way to generate VCC. But transformers add to manufacturing costs so it is less expensive to use non-isolated switching regulator architectures such as a buck or a buck-boost switching power converter to power LEDs. In a non-isolated switching regulator, the power switch couples to an inductor. Although the simplicity of an inductor as opposed to a transformer lowers costs for non-isolated switching regulators, the efficient generation of VCC at a suitably low cost becomes more challenging. For example, a “source-switching” VCC charging architecture may be used in non-isolated switching power converters in which the power switch transistor comprises an NMOS power switch transistor having its drain coupled to the inductor and its source coupled to a source voltage terminal of the controller. The gate of the NMOS power switch transistor is driven by a relatively constant voltage derived from the rectified input voltage. The controller includes a first control switch for controlling whether the source voltage terminal is grounded. If the source voltage terminal is grounded through the first control switch, the NMOS power switch transistor switches on for a power cycle. Should the first control switch turn off the source voltage terminal floats to a sufficiently high voltage such that the NMOS power switch transistor switches off. The “source-switching” designation of the architecture is thus provided the selective coupling of the source for the NMOS power switch to ground through the first control switch.
To generate VCC, the controller in a source-switching architecture includes another second control switch coupled between the source voltage terminal and a VCC terminal. When the second control switch switches on, a storage capacitor coupled to the VCC terminal is charged. The controller regulates the cycling of the second control switch to regulate VCC. Although the second control switch regulation works efficiently for relatively low power LEDs, its efficiency drops for higher-power systems such as an output power of 20 W or greater. In particular, the drain-to-source on-resistance (Rdson) for the first control switch becomes problematic as the first control switch is in the main conduction path for the inductor. Note that the first power switch is incorporated into the controller die. As the inductor current is increased to produce higher output powers, the power losses from Rdson becomes problematic for a relatively small transistor size for the first control switch. The solution for high-power source-switching is thus to increase the die space for the first control switch or replace it with an external transistor, which increases manufacturing cost.
Current alternatives to source-switching architectures also suffer from high costs or low efficiency with regard to generating VCC. For example, an external capacitor may be coupled between the drain terminal on the controller that couples to a drain of the power switch transistor and another suitable controller terminal such as an AC supply terminal. As the drain voltage toggles from high to low in each power switch cycle, the external capacitor is charged to provide a VCC source. Although such generation is very simple, its efficiency is poor as most of the charging energy is lost.
Accordingly, there is a need in the art for non-isolated switching power converters with efficient VCC generation.